Thermoplastics (TPs) are very important items of commerce. Typically they are formed into various parts and shapes by melt forming, that is melting of the TP, forming it while molten into a shape and then cooling the TP to a solid to “fix” it in that shape. In most melt forming machines, the TP is fed in the form of a pellet or granule, typically in the size range of 0.1 to about 0.6 cm (longest dimension). In order for most melt forming machines to work efficiently, it is preferred that the pellets or granules be free flowing and have a reasonably uniform size.
Many types of apparatuses have been developed to pelletize TPs. Such an apparatus should preferably produce uniform and readily flowing pellets, at low cost. One such type of pelletizing apparatus is the so-called “underwater melt pelletizer” (UMP) or cutter, see for instance U.S. Pat. Nos. 2,918,701 and 3,749,539. When a UMP is operating properly, it is capable of producing large amounts of TP pellets which are uniform and free flowing.
UMPs have been particularly useful for cutting TPs that have relatively low melting points and or do not otherwise freeze (solidify) readily. This is because as the molten TP exits the die plate it is exposed to liquid water, which is relatively cool. Upon contacting the water the TP tends to freeze rapidly. If the TP has a high melting point enough heat may be removed from the TP and also the face of the die plate which is in contact with water to also freeze some of the polymer which has not exited the die hole, thereby clogging that die hole (and presumably others in the die). This can be rectified to some extent by using heated water (which itself may be a burn hazard for the operators). Another method which has been used is insulating the die hole orifice from the rest of the die body, to minimize heat loss from the die body, see for instance U.S. Pat. No. 4,678,423. However these and other methods have not proved very effective for higher melting and/or fast freezing polymers.
U.S. Pat. No. 3,749,539 describes a UMP die whose nozzles and body may be made from “nickel 200”. This alloy is reported to have a thermal conductivity of about 70 W/m° K. However there is no insulating member at the end of the die hole.
U.S. Pat. No. 4,678,423 describes a UMP die what has nozzles which have a thermal conductivity of about 60 W/m° K or more. The thermal conductivity of the rest of the die body is not specified, the ends of the nozzles near the exit surface are thermally insulated from the rest of the die body, and the tips of the nozzles at the exit face are not insulated.
U.S. Pat. No. 4,728,276 describes an “Underwater Pelletizer” with die holes having a reverse taper. No mention is made of the effect of such die holes on a startup.
U.S. Pat. No. 4,752,196 describes a UMP die having some insulation near the tips of the nozzles. However the die described does not have a relatively highly thermally conductive path from the heat source to the inner surface of the die holes, and the insulation is arranged differently than herein.
Japanese Patent Application 5-253997 describes a die whose holes have a reverse taper. The purpose of these holes appears to be minimization of die drips and their degradation, possibly with cutters which are not UMPs.
It is therefore an object of the present invention to provide an apparatus suitable for polymer pelletization which minimizes the potential for the freezing of polymer. A feature of the present invention is the strategic incorporation of thermally conductive materials into such apparatus to facilitate polymer flow. An advantage of the present invention is the attendant savings in cost and time through more efficient and consistent operation of the UMP's equipped with such apparatus. These and other objects, features and advantages of the invention as disclosed and claimed herein will become apparent upon having reference to the following detailed description of the invention.